[Peephole] rewrite INSERT_SUBREG to SUBREG_TO_REG if upper bits zero

Restrict the 32-bit form of an instruction of integer as too many test cases
will be clobber as the register number updated.
    From %reg = INSERT_SUBREG %reg, %subreg, subidx
    To   %reg:subidx =  SUBREG_TO_REG 0, %subreg, subidx
Try to prefix the redundant mov instruction at D132325 as the SUBREG_TO_REG should not generate code.

Reviewed By: efriedma
Differential Revision: https://reviews.llvm.org/D132939